Doesn't work anymore, that's the problem. "pragma = no cache, must revalidate" and stuff like that. They are hiding the original files. Unplug and KeepVid plugins don't work at all, anymore.
I looked at the download helper from Real. It's spliting/copying the stream to a folder.
Google does it differently. They split the video into pieces. As soon as one part is cached it begins deleting what has played and then caches the next segment. No matter how fast your finger are, it isn't all there in Firefox's cache to grab.